Recently, the laying of high strength large diameter line pipe having a tensile strength of 850 MPa or more has started to be studied from the viewpoints of streamlining transport in long distance pipelines transporting natural gas and of reduction of the cost of ancillary facilities. Such line pipe is usually produced by shaping steel sheet into a tube and seam welding the abutted part such as with the UOE process, UOC process, JOE process, or bending roll process. In this case, the seam weld zone forming the joint is usually formed by submerged arc welding in the order of inner surface welding and outer surface welding. However, non-destructive inspection after outer surface welding sometimes reveals cracks in a direction perpendicular to the steel pipe axial direction at the seam weld zone, that is, so-called transverse cracks.
If using such a steel pipe where this kind of transverse crack remains in a frozen terrain, there are the danger of seasonal fluctuations in temperature causing a tensile stress exceeding the yield strength of the pipe body in the axial direction being applied and the pipe breaking and the danger of repeated application of stress causing cracks to grow, the transported fluid to leak, and a major accident occurring. Because of this, it is necessary to prevent the formation of cracks at the time of production in advance or to reliably detect cracks formed by non-destructive inspection and eliminate them.
A transverse crack in the seam weld zone is a type of embrittlement crack of a high strength material. This embrittlement crack is generally due to hydrogen. This is also referred to as a “hydrogen embrittlement crack”. If the strength decreases, this becomes harder to form. However, if reducing the strength of the seam weld zone, while embrittlement cracks become harder to occur, deformation from the seam weld zone is selectively promoted at the time of application of internal pressure leading to breakage from the weld zone in some cases. Consequently, a method of preventing hydrogen embrittlement cracks while maintaining the strength of the weld metal at the base material strength or more has become necessary.
Hydrogen embrittlement cracks depend on the hydrogen concentration, load stress, and material characteristics, in particular the strength, so it is necessary to control these to their limit values or less so that hydrogen embrittlement cracks do not occur due to the composite effect. As the method for lowering the hydrogen concentration, the method of heating to 100° C. or more, preferably 200° C. or more, after welding and holding this for exactly a suitable time, so-called “post heating”, is the method of heating the weld metal after seam welding to cause diffusion of the hydrogen to a hydrogen concentration of less than the limit where transverse cracks occur.
From this viewpoint, the technology of compositely suppressing the strength of the weld metal of a UOE steel pipe, the strength of the base material, and the welding conditions to prevent hydrogen embrittlement cracks in the seam weld zone of a high strength material is disclosed in Japanese Patent Publication (A) No. 2003-311321. Japanese Patent Publication (A) No. 2003-311321 describe that transverse cracks in the weld zone frequently occur at the preceding seam weld zone, but does not disclose specific conditions for preventing transverse cracks by suppression of the hydrogen concentration and weld residual stress.
Further, a method of welding, then quenching and tempering the steel pipe as a whole so as to prevent a drop in the toughness and solidification cracking is proposed in Japanese Patent Publication (A) No. 57-35636, but the hydrogen concentration and weld residual stress were not touched upon. In addition, as a method for relieving the residual stress, which is a cause inducing hydrogen embrittlement cracks, there are also so-called stress-relief annealing of heating the material to about 700° C. after welding or the method of striking by hammer peening to give plastic deformation to the weld zone and thereby lower the residual stress, but the effect of the relationship between the hydrogen concentration and residual stress on transverse cracks is not clear and the hydrogen embrittlement cracking resistance is not sufficiently improved. Further, these methods have to be performed immediately after welding. If considering the production process and production costs, they are not necessarily suitable methods for application to the seam weld zone.
Regarding the relationship between the amount of diffusible hydrogen diffused in the steel at ordinary temperature and released at the time of heating up to 400° C. and the hydrogen embrittlement cracking, the fact that hydrogen embrittlement cracks do not occur even in a high strength material of over 827 MPa with an amount of diffusible hydrogen of not more than 5 cc per 100 g is reported in Proceedings of IPC 2004, Oct. 4 to 8, 2004, IPC04-0585 Evaluation of Hydrogen Cracking Susceptibility in X120 Girth Welds. However, this discovery is described in regard to gas welding for welding together steel pipes by multiple passes. At weld zones in seam welding covered by the present invention, it is confirmed that hydrogen embrittlement cracks still occur even with 5 cc or less per 100 g.
Further, as points for improvement of the weld material, there is the method of causing the formation of VN or other hydrogen trap sites in the weld metal to reduce the diffusible hydrogen harmful for cracks or the method of reducing the residual stress at ordinary temperature by a low temperature transformation welding material. However, use of hydrogen trap sites is not necessarily a useful method in a high strength material. Further, the use of a low temperature transformation welding material invites a remarkable rise in costs.